In the development of microelectronics, there is an ongoing effort to reduce the size of microelectronic devices and the elements that make up the devices. As these dimensions continue to shrink, the need for alternative gate dielectric materials will become more important. Silica, having an empirical formula of SiO2 and commonly referred to as silicon dioxide, has conventionally been the material of choice for gate oxides because it readily forms on a silicon substrate by oxidation of the silicon. At thicknesses below about 2 nm, however, leakage currents through silica films become unacceptably high during normal operating conditions. Replacement of silica with materials having a higher dielectric constant (xe2x80x9chigh-K materialsxe2x80x9d) has been investigated. Films of high-K materials, however, have typically been plagued by poor interfaces and high cost of production.
One class of high-K films is the metal oxide family of general empirical formula MOx (where xe2x80x9cMxe2x80x9d is a metal and x is from 0.01 to 4). Metal oxide films can be prepared through a variety of techniques. For example, vapor deposition of a metal oxide can be accomplished by treatment of the surface with a vaporized metal (i.e. physical vapor deposition). Vapor deposition of a metal oxide can also involve treatment of the surface with a vaporized metal alkoxide of the general formula M(OR)y, where y is from 1 to 8, and R is an alkyl group. This process is referred to as chemical vapor deposition (CVD). xe2x80x9cAlkylxe2x80x9d refers to a substituted or unsubstituted, straight, branched or cyclic hydrocarbon chain containing from 1 to 20 carbon atoms. The chemisorbed layer formed is then treated with an activating agent such as an oxidizing agent or water, or by exposure to heat or light to form the MOx film. See, for example, Tada, H. Langmuir, 11, 3281 (1995); and Zechmann, C. A. et al. Chem. Mater., 10, 2348 (1998).
One particularly interesting high-K material is zirconia, commonly referred to as zirconium oxide. The term xe2x80x9czirconiaxe2x80x9d is defined herein as a substance having an empirical formula of ZrO2, and which may include trace amounts of impurities such as hafnium, water, or hydrocarbons. Zirconia has good performance characteristics as a dielectric gate material due to its stability on silicon.
In addition to vapor deposition processes, a conventional method of forming films of zirconia or other metal oxides on a semiconductor such as silicon is atomic layer deposition (ALD). The ALD process involves a high temperature condensation of evaporated metal-containing precursors on the semiconductor surface, followed by a hydrolysis reaction with water, and then repeating the condensation and hydrolysis one or more times. Anhydrous zirconium alkoxides are stable at ambient temperature and form zirconia through a series of hydrolysis (1) and condensation (2) reactions:
Zrxe2x80x94O-alkyl+H2Oxe2x86x92Zrxe2x80x94OH+alcoholxe2x80x83xe2x80x83(1)
Zrxe2x80x94OH+Zrxe2x80x94O-alkylxe2x86x92Zrxe2x80x94Oxe2x80x94Zr+alcoholxe2x80x83xe2x80x83(2).
This approach of using alternating surface reactions can be employed in a CVD chamber to grow zirconia films on a substrate using Zr[OC(CH3)3]4 at temperatures ranging from 150xc2x0 C. to 300xc2x0 C. Hydroxyl (xe2x80x94OH) groups on the surface of the substrate are believed to provide initial sites for condensation reactions. The reactions between zirconium alkoxide groups and the hydroxyl groups yield a single layer of chemisorbed zirconium alkoxide according to reaction (3):
Surf-OH+Zr(O-alkyl)4xe2x86x92Surf-Oxe2x80x94Zrxe2x80x94(O-alkyl)3+alcoholxe2x80x83xe2x80x83(3).
The adsorbed layer [xe2x80x94Zrxe2x80x94(O-alkyl)3] is xe2x80x9cprotectedxe2x80x9d from multilayer formation by the remaining unreacted xe2x80x94O-alkyl groups. The zirconium alkoxide adsorbed on the surface is then xe2x80x9cdeprotectedxe2x80x9d through alkyl group elimination by hydrolysis according to reaction (4):
Surf-Oxe2x80x94Zrxe2x80x94(O-alkyl)3+3H2Oxe2x86x92Surf-Oxe2x80x94Zrxe2x80x94(OH)3+3 alcoholxe2x80x83xe2x80x83(4).
A second exposure to zirconium alkoxide results in further surface condensation as in (3). Through repeated condensation-elimination cycling, a robust zirconia film is formed layer by layer. See, for example, Kukli, K. et al. Chem. Vap. Deposition, 6 (2000), p. 297.
The conventional methods of forming metal oxide films, including zirconia and HfO2 films, on semiconductors have met with mixed success. Disadvantages of these methods include the high cost of using elevated temperatures and/or reduced pressures for depositing the metal oxide precursors on the semiconductor surface. Also, due to the elevated temperatures used, a thick oxide interface, containing silicon and the metal from the metal oxide being formed, can be present between the silicon substrate and the desired high-K metal oxide film. Irregularities in the surface of the metal oxide films and/or in the interface between the film and the semiconductor can also be problematic.
It is thus desirable to provide thin metal oxide films on semiconductors using lower temperature processes. Preferably, these metal oxide films do not contain significant surface irregularities and can be formed reproducibly. High-quality, ultrathin metal oxide films would likely be useful as gate dielectrics in semiconductor structures, as dielectrics in metal oxide semiconductor capacitors, and as barrier layers in semiconductor processing.
In a first embodiment of the invention, there is provided a method of making a semiconductor structure, comprising contacting a surface of a semiconductor with a liquid comprising Zr4(OPrn)16 to form a modified surface; activating the modified surface; and repeating the contacting and activating to form a layer of zirconia on the semiconductor surface.
In a second embodiment of the invention, there is provided a method of making a semiconductor structure, comprising obtaining a liquid comprising analytically pure Zr4(OPrn)16; contacting a surface of a semiconductor with the liquid in an inert atmosphere to form a modified surface; rinsing the modified surface; hydrolyzing the modified surface with an aqueous liquid comprising n-propanol to form an activated surface; drying the activated surface; repeating the contacting, rinsing, hydrolyzing, and drying to form a layer of zirconia on the semiconductor surface; and heat treating the semiconductor comprising the layer of zirconia.
In a third embodiment of the invention, there is provided a semiconductor structure comprising a semiconductor substrate and a layer comprising zirconia on the substrate, the layer having an equivalent oxide thickness of not more than 2 nanometers. The semiconductor structure has a leakage current less than 0.002 A/cm2 when subjected to a potential of 1 volt.
In a fourth embodiment of the invention, there is provided a semiconductor substrate comprising a first surface and a second surface; a layer comprising zirconia on the first surface; a first layer of a conductor on at least a portion of the zirconia layer; and a second layer of a conductor on at least a portion of the second surface. The capacitor has a leakage current less than 0.002 A/cm2 when subjected to a potential of 1 volt in accumulation.